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BRITAIN has not always been free of rabies. The Victorians tried to
cope with an outbreak by passing laws that required all dogs to be muzzled.
To the consternation of the citizenry, policemen shot unmuzzled dogs in
the streets. The advent of Louis Pasteur’s vaccine led to calls for mass
vaccination as an alternative, but in the end muzzling and the ‘shoot on
sight’ policy worked. Since then, Britain has remained free of rabies, except
for a few years after the First World War, when returning servicemen briefly
reintroduced the disease. Again the disease was eliminated by culling infected
animals.

Now the building of the Channel Tunnel has heightened fears that the
disease may once again become endemic in Britain. Yet the proliferation
of marinas on Britain’s south coast poses a far bigger risk and certainly
a much bigger problem for surveillance. As rabid foxes move closer to the
French coast, irresponsible weekend yachters who take pets across the Channel
may breach our rabies defences.

Concern now centres on what we should do were a rabid dog to enter the
country and pass on the disease to Britain’s thriving population of urban
foxes. How could we stop its spread? In continental Europe, biologists are
now trying to vaccinate foxes to halt the spread, and there are signs that
such an approach will work in much of continental Europe (see ‘How Europe
is winning its war against rabies’, ¿ìè¶ÌÊÓƵ, 26 May). But our research
at the University of Bristol suggests that this approach would not work
in Britain, and instead we might be forced to kill foxes living in a particular
area to stop the epidemic.

If rabies does reach Britain, the problem will be very different from
that in Europe. Rabies in Europe is endemic and is spreading in waves on
a front 2000 kilometres long. But Britain would experience a ‘point source’
infection. This means that if the authorities detect the disease early,
and set up a means of control quickly, only a few foxes in a relatively
small area might be infected. Once these are eliminated, there would be
no source of reinfection.

Yet in other ways rabies in Britain would be more difficult to deal
with than in Europe. More than 80 per cent of the British population lives
in cities of more than 50 000 people, so a smuggled pet would probably end
up in an urban area. It could then pass on the disease to both domestic
and stray dogs and cats and, if the red-fox strain of the virus was present,
to urban foxes . British cities may harbour foxes at densities five times
those in rural Britain, and much higher than those in many parts of Europe.

At Bristol, we have been trying to identify the problems posed by controlling
rabies in an urban population of foxes. One approach has been to simulate
how rabies might spread in a particular city, and to examine how effective
different strategies of control might be. We can now accurately predict
the number and distribution of foxes in any British city, based on such
things as the patterns of leafy, middle-class residential areas. Foxes thrive
in such areas: 70 per cent of their diet consists of handouts from local
residents who get great pleasure from feeding ‘their’ foxes.

With this model, we attempted to chart the spread of rabies in populations
of urban foxes (see Figure). In our simulations rabies became established
in the fox population even if only one family group, some five foxes, was
infected. The high density of foxes in many of Britain’s cities means that
the disease could spread easily once one rabid animal has infected a fox.
Calculations suggest that if this happens the rabies epidemic could be contained
only if 92 per cent of the fox population in the urban area was prevented
from spreading the disease, either by culling or vaccination. In the simulation
described in the figure, rabies was eliminated a year after it was introduced
by a cull removing 92 per cent of the animals.

Several European countries have begun to vaccinate wild foxes with treated
bait, rather than putting poison in the baits. But this normally reaches
only about 70 per cent of the foxes. In Europe, this is sufficient to eliminate
the disease because in these lower-density populations such a level of vaccination
leaves too few susceptible foxes to maintain the infection.

Four years ago, Roy Anderson of Imperial College London, writing in
Nature (vol 322, p 304), suggested that in Britain we would need to vaccinate
roughly 95 per cent of the fox population to achieve successful control.
Anderson felt that vaccines were unlikely to control rabies in British cities.
Field trials in Bristol have confirmed his fears: we and our colleagues
have found that in a single baiting trial it is very difficult to get more
than 40 per cent of the foxes to take a bait. So oral vaccines would achieve
little.

Why is it so difficult to get foxes to take baits? The wide variety
of alternative sources of food in urban areas is part of the problem. Moreover,
at high densities, foxes live in family groups, and our preliminary trials
suggest that only certain members of the group take baits. So a succession
of baiting campaigns would probably have to be launched in the hope that
in time it would reach most of the fox population.

In this situation, poisoning has advantages over vaccination. With baits
containing a vaccine, the same foxes could be taking the bait on each campaign,
so even a series of campaigns would be unlikely to reach a very high proportion
of the foxes. But with poisoning, the first foxes that take the bait are
removed, increasing the chances of some of the other foxes taking baits
next time.

A poisoning campaign has certain other advantages. If rabies reaches
Britain, we may know the source of the outbreak, but equally we may not.
Dealing with an infection of known origin is comparatively easy. But what
if the first indication of a rabies epidemic is the discovery of a rabid
fox, rather than the smuggled pet? How will we know whether this is the
first rabid animal? The disease may have been present for some time, and
spread many kilometres. Then, the first rabid fox discovered may not be
the focus of infection but might be an animal that has travelled a long
way from the main outbreak. It may sound improbable that a rabid fox would
escape detection, but researchers estimate that in Europe perhaps only 1
in 10 rabid foxes are discovered.

Such imponderables make planning a control strategy complex. The great
advantage of a culling strategy is that there is a supply of corpses to
check for infection. These can be used to monitor the situation rapidly
and show how well established the disease has become, and how far it has
spread.

With a vaccination campaign there is no such feedback. Once the baits
have been put out, the only way to measure success is to catch live foxes
and take blood samples – a time-consuming procedure. When a fox is sighted
in the area, no one can tell if it is vaccinated or not. With culling, it
is easy to see whether the control operation is working: every fox that
remains has been missed, and so is potentially a rabid animal.

Culling would also help to reassure members of the public. Urban foxes
are often tame and frequently approach people without signs of fear. Some
foxes already cause people concern; with the arrival of rabies, they would
cause consternation. No one would know if the fox in their garden was vaccinated
and just friendly, or not vaccinated and possibly rabid.

So although killing foxes to control rabies is not a pleasant option,
it seems to be the best way to deal with rabies in Britain. In such cases,
vaccines should be used only as an adjunct to culling or if culling fails
and the disease becomes endemic, as it has in continental Europe.

* * *

How sentimentality can have deadly results

BRITAIN remains rabies-free by imposing strict quarantine regulations
on the import of many species of mammals. In 1989, 5915 dogs, 3220 cats
and 140 327 other mammals were legally brought into Britain under licence.
But in the same year, 38 dogs, 28 cats and 93 other mammals were known to
have been smuggled in – and this probably represents a small fraction of
the smuggling that takes place. If one such animal was rabid, it could set
up a focus of infection.

Different strains of the rabies virus thrive in different parts of the
world. In much of Africa and Asia, the virus is adapted to live in the domestic
dog, and this strain of virus, even if introduced into Britain via a smuggled
pet, would probably remain confined to domestic animals, and so would be
much easier to control. In Europe and parts of North America, the virus
lives primarily in the red fox, but it can infect other species, including
dogs and cats. So a smuggled pet infected with the fox strain of the virus
poses the greatest problem – it could easily establish an infection among
wild foxes. This means customs authorities must worry most about pets smuggled
from Europe or parts of North America.

There is one other possible route – an infected bat. In the past five
years, several European bats have been found to be rabid. Foxes can apparently
catch rabies from bats by eating or even sniffing infected carcasses. However,
it seems that the viral strain from European bats may kill a terrestrial
mammal but not spread to others, so bat rabies in Europe is unlikely to
be a big problem. Moreover, in recent years there have been fewer cases
in Europe. As a precaution, because bats do migrate between Britain and
continental Europe, Arthur King, at the Central Veterinary Laboratory, Weybridge,
routinely screens bat carcasses for rabies. So far, the 652 screened between
1986 and 1989 have been negative.

Stephen Harris and Graham Smith are in the department of zoology at
the University of Bristol.